The invention relates to a device for the inspection of workpiece surfaces using a sensor, such as, e.g., a camera or an eddy current sensor. The invention relates in particular to a device which is suitable to hold the sensor at a constant distance to the workpiece surface and to rapidly correct a change in distance. The invention further relates to the inspection or measurement of material properties of strip materials.
Systems are known in the art to make it possible to inspect a surface of a workpiece or strip material. For this purpose, sensors must be kept precisely at a predetermined position. When, e.g., an optical inspection is to be executed by an optical system having a very shallow depth of field, the distance of the objective of this system and the workpiece surface must be kept constant as the optical system is guided across the workpiece surface. Such systems are known from the documents WO94/18643 and WO01/23869 A1. Comparable requirements apply also to other sensors and other measuring or inspection processes such as, e.g., for capacitive or inductive inspection processes.
As the required accuracy increases, the demands on the mechanical design of the measuring arrangement increase as well. This is especially true, when the measurement accuracy depends on the compliance with a defined distance between the sensor position and the workpiece surface. When, e.g., during monitoring of strip materials, a sensor is to be moved in close proximity across the surface of the strip material transversely to the direction of movement of the strip, rigid traverse bars are used for this purpose.
Since the strip material normally oscillates slightly during movement, measuring errors occur. Also, measuring errors are encountered, when the strip passes at the measurement position over a roller, because rollers can be out-of-round. When the strip material to be inspected involves, e.g. a steel sheet with a hard surface, the sensor may be coupled in such a case with a carriage which has a support wheel rolling on the surface of the steel strip, so that the sensor has the same distance to the steel strip surface at all times.
There are, however, a number of applications in which the strip material must be inspected during a treatment process, e.g. a coating process. The strip material passes hereby e.g. a drying tunnel in which a constant temperature and a uniform temperature distribution must be maintained. In this case, the afore-described device cannot be used for two reasons: As a result of the direct mechanical contact of the support wheel with the strip material, the sensitive surface of the strip material could be damaged, on one hand, and the surface temperature of the strip material could be altered at the contact point, on the other hand.